When a vertebra is broken or crushed it is frequently necessary to ablate the body of the crushed or broken vertebra or vertebrae. In order, however, to prevent the spinal column from collapsing with damage to the fragile spinal cord running in the vertebral foramen forward of the vertebral body, it is necessary to employ an implanted spacer. This device is braced vertically between the bodies of the adjacent vertebra and holds them apart at the desired spacing. It may even serve to distract two vertebrae which have become too closely spaced due to crushing of a vertebra or disk.
In U.S. Pat. No. 5,571,192 a spinal implant is described for engagement in a space left by ablation of a vertebral body between a pair of adjacent vertebrae. It has a tubular center element extending along an axis and a pair of end elements. The center element is formed with upper and lower screwthreads of opposite hand and with a plurality of radially throughgoing apertures. The upper and lower tubular end elements are each formed with a plurality of radially throughgoing apertures, each have a circular-section inner end threaded onto a respective one of the screwthreads, and each have an outer end adapted to bear on a respective one of the adjacent vertebrae.
Thus such an implant can be set in an area where the body or bodies or one or more vertebra have been ablated. The length of the implant is then increased by rotating the center element to force out the end elements and bring their outer ends into solid engagement with the confronting vertebral surfaces. The screwthreads offer sufficient mechanical advantage so that the system can even be used to distract the vertebrae, as is frequently necessary in the event of a crushing injury. The tubular elements of the implant can be filled with bone cement and/or bone fragments to ensure that the implant becomes anchored in place in living bone. Since the outer elements surround the screwthreads of the inner element, once installed the screwthreads will be largely covered so that their sharp edges do not impair healing.
Such an arrangement serves very well to maintain the axial spacing between the vertebrae it is installed between, but must normally be used in conjunction with a separate bone plate and or external dorsal appliance secured by bone screws to the flanking vertebrae. This extra equipment is needed to prevent any relative lateral shifting of the vertebrae and to prevent the implant from moving out of position.
Not only is the plate or appliance difficult to install, but it is extremely inconvenient for the patient whose wound must often be left open. In addition the plate or appliance often block access to the implant so if same needs to be adjusted, for instance lengthened, it must be removed and reinstalled.